A. Field of the Invention
This invention relates, in general, to a production method of a substrate, particularly in plastic material, coated with a mesoporous layer and coated substrates obtained in this way.
More particularly, this invention relates to a production method of a transparent substrate, preferably in a transparent plastic material, such as an optical or ophthalmic lens or lens blank, coated with a mesoporous layer, this mesoporous layer generally being a low refractive index layer, that is a refractive index n≦1.50, preferably ≦1.35 (λ=633 nm, T=ambient (20-25° C.).
B. Description of Related Art
The preparation of mesoporous layers has already been described in the state of the art.
Mesoporous materials are conventionally defined as materials containing pores of a diameter comprised between 2 and 50 nm.
The conventional method for preparing mesoporous layers consists in preparing a slightly polymerised silica sol, from a precursor such as a tetraalkoxysilane, particularly tetraethoxysilane (TEOS), said sol also containing water, an organic solvent, generally polar such as ethanol and a surface active agent, usually in an acid medium.
The concentration of surface active agent is at this stage considerably lower than the critical micellar concentration.
The solution is then deposited onto a substrate. During the deposition, the organic solvent evaporates, which increases the content of water, surface active agent and silicon dioxide in the film, and then the critical micellar concentration is reached. Since the solvent medium is very polar, the molecules of the surface active agent cluster together; the micelles having their polar head groups directed toward the solvent.
The silicon dioxide, which is also very polar, surrounds the micelles and so a structuring of the film is obtained.
As the evaporation continues, the micelles may change shape and organise themselves into more or less organised structures, until the film dries.
The porous material is obtained after the surface active agent is eliminated.
This elimination can be done by calcination (heating at a temperature of the order of at least 400° C.), or by more moderate methods (extraction using solvents, UV/ozone, plasma).
Instead of silicon dioxide, it is possible to use other precursor oxides such as metallic precursors, for example ones based on titanium, niobium or aluminium.
The mesoporous layers disclosed in the state of the art generally present high levels of porosity, greater than 40%, and these pores are filled with air, and have the ensuing properties: low refractive index and low dielectric coefficients, particularly.
The preferred applications of these films are in the field of electronics.
One of the disadvantages of these films is their low stability in presence of an atmosphere with a high degree of moisture. These films have a tendency to absorb water over the course of time, which modifies their initial properties.
This question of the stability of optical properties of mesoporous layers is particularly important if they are to be used in optical applications, particularly in antireflection stacks because, contrary to applications in the field of semi-conductors, where a variation of the dielectric coefficient within predefined limits can be envisaged without affecting the operation of the semi-conductor, very small variations in the refractive index have an immediately noticeable consequence in the field of optics, for example in altering the colour and the performance of an antireflection coating.
Many techniques can be found in prior art that aim to improve the stability of mesoporous films, in particular their dielectric properties.
These techniques include a step of calcination of the film at a temperature of 350-400° C.
The U.S. Pat. No. 5,858,457 discloses a method for preparing mesoporous films obtained from a sol comprising TEOS, ethanol, water and hydrochloric acid. The surface active agent used is cetyltrimethylammonium bromide CH3(CH2)15N(CH3)3Br (CTAB).
After being deposited, the layers are calcined at 400° C.
The patent mentions that other techniques (by solvent extraction) may be used.
The patent indicates that these films can be used as optical coatings and that they can also be used as antireflectives, without giving any more details about the way the mesoporous layer could accomplish this antireflection function.
The films obtained according to the technique of this patent have proved to change rapidly with time, in particular in damp atmospheres, and are not useable in practice.
The patent application WO 03024869 discloses a method for preparing mesoporous layers from a sol obtained by reaction between TEOS and an alkoxysilane substituted by an alkyl group (such as methyltrimethoxysilane) in a molar ratio of less than 3/1 and preferably less than 3/2.
This method improves the stability of the layers by limiting their ability to absorb water, while modifying relatively little their mechanical properties.
The layers obtained are used as insulating layers in semi-conductors or for their optical properties in transmission filters.
The document US 2003157311 discloses the preparation of mesoporous layers with low dielectric constants from precursor sols prepared with TEOS as the first precursor agent and at least one organofunctional alkoxysilane, as a second precursor agent, such as a fluoroalkoxysilane or an alkyltrialkoxysilane, for example methyltriethoxysilane.
In general, the document US2003157311 discloses formulations wherein the molar ratio first precursor agent/second precursor agent can vary from 0.05 to 1.
The hydrolysis of alkoxysilanes is carried out sequentially and preferably in the presence of an organic acid such as acetic acid and necessitates the ageing of the sol obtained (typically 5 day).
After depositing the sol containing the two precursor agents, it is necessary to proceed to a calcination step (namely at 425° C.).
The document also describes, for low ratios of the two precursors above (5%), an post-treatment of the film after calcination, by treating with hexamethyldisilazane (HMDS), applied in the liquid phase, followed by a step of heating to 350° C.
The aim of this post-treatment is to limit the quantity of water adsorbed into the pores of the mesoporous material, in order to maintain a low dielectric constant.
Such a method does not appear to be adapted for treating organic substrates which would be degraded by calcination temperatures.
The document WO 9909383 also describes the post-treatment of a mesoporous TEOS gel with trimethylchlorosilane, after a solvent exchange has been carried out on the gel.
After the post-treatment, the gel is put back into solution under the effect of an ultrasound treatment, deposited on a substrate then calcined for one hour at 450° C.
The final mesoporous material obtained is used as a thermal insulator.
All these methods present a calcination step at a high temperature (of the order of 400° C.), which makes them unsuitable for the preparation of mesoporous films on organic substrates, and particularly transparent organic substrates such as optical or ophthalmic lenses.